A common technique for combining many independent signals so that they can be transmitted over a common communication channel is frequency division multiplexing (FDM).
The common communication channel contains a bandwidth which exceeds the required bandwidth of signals to be transmitted. Therefore individual signals to be transmitted can be multiplexed up in frequency such that the spectrum of the signals may fall within the common communication channel bandwidth. The channel carrier for each signal to be transmitted is chosen such that the resultant modulated signals occupy adjacent, nonoverlapping frequency bands or channels. The composition signal made up of the sum of individual modulated signals, is then transmitted within the common communication channel bandwidth.
Frequency division multiplexing (FDM) is typically used in satellite broadcasting. A common and serious problem in frequency division multiplexing (FDM) is intermodulation interference. Intermodulation interference is due to the amplitude nonlinearity of high-power amplifiers, such as travelling-wave tube amplifiers or klystron amplifiers. In order to derive the maximum power from the amplifiers in a satellite the amplifiers are operated near their saturation regions. The effect of intermodulation interference is to produce signals at a frequency which is the sum or difference of multiples of two or more original or desired frequencies. For example, the mixing of signals at frequencies f.sub.1 and f.sub.2 might produce energy at the frequency f.sub.1 +f.sub.2. This derived signal would interfere with a signal intended to be transmitted at the frequency f.sub.1 +f.sub.2.
Among all intermodulations, the third order is the most severe. Hence there is a need to provide an apparatus and method to plan the frequency of the various signal carriers to be transmitted over a predetermined bandwidth while minimizing third order intermodulation.
Prior systems for adjusting a frequency plan concentrated in intermodulation-free carrier frequency plans or assignment. Since an intermodulation-free carrier frequency plan is rarely achievable, it is more practical to search for a carrier frequency plan which minimizes the worst intermodulation interference level across the channels.
A method for deriving sub-optimal carrier frequency plan for large number of channels has been suggested by Okinaka, et al., Intermodulation Interference-Minimum Frequency Assignment for Satellite SCPC System, IEEE Trans. Commun. Vol. 32 No. 4, pp. 462-468 (April 1984). According to the article the system, after an initial frequency plan is set, successively deletes a channel such that a large reduction in the worst intermodulation interference results. The system then inserts a channel in a new frequency location which produces a smaller worst intermodulation interference result. The process of deletion and insertion is continued until no reduction in the worst intermodulation interference results.
One effect of the Okinaka, et al. approach is the reduction of the intermodulation computation. However, this approach can only find a sub-optimal plan whose performance may be poor. For a satellite system when the number of channels per transponder is not large (for example, 10), it is possible to search more sub-optimal frequency plans and select a plan with the best performance.
The problems of determining an optimal carrier frequency plan is even more difficult when individual signals to be transmitted occupy different bandwidths. Furthermore, for systems where the channels occupy a significant portion of the transponder bandwidth the problem becomes even more acute.
Therefore, there is a need for an optimal carrier frequency planning system wherein carrier frequencies are derived and utilized to minimize intermodulation noise.